Idea of element co-adaptation of construction ergonomics claims the elements of construction ergonomic system mutual fitness. Flexibility of construction system embodies the element co-adaptation. Flexible ergonomic principle indicates that construction system design and update must make the system to have and keep the function of fit in with and conversion of noise to meet the needs of flexibility of construction system. Principle of factors fit in with system expresses the relations between construction ergonomic system and its factors. The principle claims any one of the factors should co-ordinate the system, and that is the precondition of system flexibility. Principle of factors mutual fitness expresses the relations between the factor and factor of system. It voices that any one of the factors (and fine factors) should adapt to other ones and keep on the adaptation. The paper names the last two principles combined as “fit flexible principle” and proofs it by using system evolutionary equation through defining the conception of adaptable coefficient of factors. Findings of the research have corrected the ergonomic system design idea that based on cybernetics as well as the optimizing idea of construction resource that based on static thinking.